The invention relates to a separation-column unit for a gas chromatograph, having a support plate, which on one side contains a groove which runs in a predetermined continuous line and on this side is covered by a cover plate. The invention also relates to a process for producing a separation-column unit of this type.
A separation-column unit of this type is known from U.S. Pat. No. 5,583,281. The production of this separation-column unit is described in U.S. Pat. No. 5,575,929. In this process, a groove which runs in the shape of a spiral and is of semicircular cross section is formed in a support plate made from silicon by isotropic etching. In the same way, a channel, which is likewise semicircular in cross section, is etched into a cover plate. Then, the two plates are connected to one another, the groove in the support plate and the channel in the cover plate complementing one another to form a spiral-shaped capillary which is circular in cross section. On the inside, the capillary can be coated in a known way with a separation phase, in order to form the separation column.
In practice, however, separation columns are designed as glass capillaries which are round in cross section and the inner surface of which is coated with the separation phase. For economic reasons, there is no particular sense in designing such separation columns as a unit comprising a support plate and cover plate, since, apart from resulting in a smaller structure, this does not lead to any discernible technical benefits. Rather, the separating capacity of the separation column may be reduced considerably if, on account of geometric flaws, the groove in the support plate and the channel in the cover plate are not precisely congruent and therefore the cross section of the capillary formed is not circular.
Separation columns with a very small internal diameter of the capillary, which are known as narrow-bore columns, are distinguished by a high separation capacity, the optimum level of which is reached at a defined velocity of the sample flowing through the separation column. A further advantage of such separation columns is that at higher flow velocities of the sample, the separation capacity is reduced only slightly. As such, the analysis time can be shortened drastically without a significant deterioration in the separation capacity. This is of great importance in particular for on-line chromatography, on account of it being directly incorporated in the industrial control process. By contrast, a drawback is the low load from the amount of sample to be separated which these narrow-bore columns are able to bear. This is because, if the amounts of sample increase the separation phase rapidly becomes overloaded, there is an enormous deterioration in the separation capacity.
The invention is therefore based on the object of providing a separation column which is distinguished both by a high separating capacityxe2x80x94even at relatively high flow velocities of the samplexe2x80x94and by a high load-bearing capacity with regard to the amount of sample.
According to the invention, the object is achieved by the fact that, with the separation-column unit described in the introduction or the process for its production, the depth of the groove is or is designed to be greater than its width. As a result, the surface area of the separation column which is active in the chromatographic separation is enlarged with respect to its volume, so that both the separation capacity and the load-bearing capacity of the separation-column unit are improved. Because of this it is possible, for example for a given separation capacity, to reduce the length of the separation column compared to conventional separation columns which are circular in cross section, so that the overall size of the separation-column unit can be reduced accordingly.
Although it is conceivable for the glass capillary in conventional separation columns to be compressed by pressing or rolling, in order in this way to produce an elongate cross section, the cross section which is produced in this process varies over its length. This is because the compression in the center of the cross section is greater than at the ends of the cross section, on account of the resistance to deformation being lower in the center. Consequently, the flow velocity of the sample and therefore the chromatographic separation vary over the length of the cross section. Thus, the so-called peak, which is recorded at the exit from the separation column as a curve illustrating the concentration of that component of the sample, which is to be detected over time, becomes wider, thereby reducing the separation capacity. Since, in the separation-column unit according to the invention, the groove,which forms the separation column is not formed by material deformation, but rather by removal of material, it is possible to attain a cross section of the separation column which is particularly uniform over the depth of the groove, i.e. the length of the cross section. The results is that the theoretically elevated separation capacity is also achieved in practice.
It is also known to increase the load-bearing capacity of a separation-column unit while maintaining a constantly high separation capacity by connecting a plurality of conventional separation columns of small diameter in parallel. In this case, however, the problem arises that if the distribution of the sample stream to the individual separation columns is not completely uniform and if there are very minor changes in the diameter and in the coating of the separation columns, the peaks have different running times. The different running times, after they have been brought together at the outlet from the separation-column unit, widen the total peak and therefore reduce the separation capacity. By contrast, in the separation column unit according to the invention, the precise production and coating options which are specified in more detail below allow much higher separation capacities to be achieved while maintaining the high load-bearing capacity.
In the separation-column unit according to the invention, the depth of the groove is preferably at least three times greater than its width, the width lying in a range between 100 and 150 xcexcm and the depth lying in a range from 500 to 600 xcexcm.
In order to achieve a uniform velocity distribution of the sample flowing through the separation column even in the region of the base, the groove preferably has a rounded, in particular semicircular cross section in that region. For the same reason, the cover plate may, on its side facing toward the support plate, contain a channel which is congruent with the path of the groove and has a cross section which is likewise rounded, in particular semicircular. In this case, it is to be assumed that the absence of the channel has less of an effect on the separation capacity than if the channel and groove are not uniformly congruent.
On the inner surfaces facing toward the groove, the support plate and the cover plate may be covered in a known way with a separation phase, in order in this way to effect the chromatographic separation. As an alternative or in addition, the support plate and the cover plate are porous on the inner surfaces facing toward the channel, the adsorption capacity of the inner surfaces and therefore their separation capacity being adjustable by means of the level of porosity.
To be able to set different separation-column lengths depending on the type of gases to be measured using the same separation-column unit, the support plate and/or the cover plate may contain continuous holes which run perpendicular to the plane of the plate and open into the groove at the ends of the groove and at predetermined intervening points. The holes in this case form the sample inlet and sample outlet for the intervening section of the separation column. However, instead of setting different separation-column lengths, it is preferably possible to change the flow velocity of the sample through the separation column which, as mentioned in the introduction, at relatively high velocities does not cause any significant deterioration to the separation capacity.
The groove may, for example, be designed in the form of a single spiral in the support plate. In this case, at least the inner end of the spiral leads outward through an opening in the support plate or cover plate. To avoid abrupt changes in cross section or flow diversions in the region of the sample inlet or outlet, the groove in the support plate preferably runs in the form of a double spiral, the groove emerging laterally from the support plate at both ends of the double spiral.
Alternatively, the groove runs in meandering form and emerges laterally from the support plate at both ends. Moreover, this has the advantage that, in addition to facilitating production, the path for the sample as it flows through the separation column does not cover a longer distance on one side of the groove than on the other side, as is the case with a groove in the shape of a spiral.
The groove may be formed in the support plate by anisotropic etching. This preferably takes place by dry etching, in particular reactive ion etching (IRE) in silicon. In this way, it is possible to form very steep groove flanks, so that the cross section of the groove and therefore of the separation column over the length of the cross section varies only to a negligible degree. Alternatively, the groove may be made in the carrier plate by means of a laser, which likewise results in very steep groove flanks. With a modified removal of material, for example a different etchant or etching process, such as isotropic etching, or a different laser wavelength, it is possible to form the base of the groove with a rounded, in particular semicircular cross section. This can also be achieved by forming the support plate from a first layer of material and a second layer of material, the thickness of the first layer of material corresponding to the depth of the groove which is to be formed and the second layer of material serving as a stopping layer for the groove-forming removal of material in the first layer of material. As a result, in particular a uniform depth of the groove over the length of the separation column to be formed is achieved. The base of the groove is preferably formed inside the second layer of material, which in terms of the material used is particularly suitable for forming recesses of semicircular cross section therein. A similar statement also applies to the cover plate, in which preferably a channel which is congruent with the path of the groove in the support plate and has a rounded, in particular semicircular cross section, is made. In this case, the base of the groove in the support plate and the channel in the cover plate can be formed by rinsing the groove with an etching solution when the cover plate is resting on the support plate, thus ensuring that the groove in the support plate and the channel in the cover plate are exactly congruent.